The present invention in accordance with certain embodiments, relates to thermographic nondestructive testing techniques for determining the thickness of a coating on the surface of an object
In general, infrared (IR) transient thermography is a versatile nondestructive testing technique that relies upon temporal measurements of heat transference through an object to provide information concerning the structure and integrity of the object. Heat flow through an object is substantially unaffected by the microstructure and the single-crystal orientations of the material of the object, therefore, an infrared transient thermography analysis is essentially free of the limitations this creates for ultrasonic measurements. Additionally, transient thermographic analysis approach is not significantly hampered by the size, contour or shape of the object being tested and, moreover, can be accomplished ten to one-hundred times faster than most conventional ultrasonic methods, particularly when testing objects with large surface areas.
Conventionally, an infrared (IR) video camera has been used to record and store successive thermal images (frames) of an object surface after heating it. Each video image is composed of a fixed number of pixels, which may be defined as a pixel array, whereby each pixel represents a small picture element in an image array or frame. Each pixel corresponds to a rectangular area, called a resolution element, on the surface of the object being imaged. Because, the temperature at each resolution element is directly related to the intensity of the corresponding pixel, temperature changes at each resolution element on the object surface can be analyzed in terms of changes in pixel contrast.
One known contemporary application of transient thermography is the ability to determine the size and relative location (depth) of flaws within solid non-metal composites; another application of transient thermography is for determining the thickness of metal objects. Some attempts have been made to measure the thickness of insulative coating as well. These include modeling techniques where the insulative coating thickness may be obtained by fitting the coating data to a model and comparing it with known thickness standards. Unfortunately, these techniques either include point-by-point measurement of the coating thickness, and therefore take time and are complex computationally or require the presence of a coating thickness standard in the image, which may not be possible or is unfeasible. Another aspect to thickness measurement for insulative coatings is that as the coating ages the thermal conductivity of the coating changes and affects the thickness measurement of the coating.
Using thermal conductivity as one factor in determining coating thickness has been achieved. The method includes obtaining a respective time-temperature response for an insulative coating and for a substrate, where the insulative coating is disposed on the substrate. The method also includes measuring a delta log value and measuring an inflection point value from the respective time-temperature response for the coating and for the substrate. These values are described in more detail with reference to equations herein below. One or more coating characteristics may be calculated using the delta log value or the inflection point value. A thermal conductivity value and a coating thickness value using both of the coating characteristic values is then possible.
However, the method is limited as the calculation is done at a specific point along the surface of the coated part. The ability to analyze variations in the coating thickness along a large surface area or geometrically complex parts is limited. Therefore, there is a need for a technique that can measure quantitatively, the absolute thickness for a coating over a large or varied surface area.